1. Field of the Invention
This invention relates to optical parts with surfaces for incident and emergent light.
2. Description of the Prior Art
In the art, optical parts such as lenses and prisms can be made generally through steps as of grinding, lapping, polishing, and finally smoothing as mirror surface.
In the following will be given the description by an example of manufacturing process of a gradient index lens array unit.
As shown in FIG. 1, a lens array unit 1 is constructed of a pair of upper and lower opposed plates 3, a pair of lateral oppossed spacers 4 (these being made of glass fiber-reinforced plastic (FRP), and a lens array surrounded and held by them. The lens array is composed of a number of glass rod lenses 2 arranged parallel in their optical axes and in at least one row, and each adjoining to other. In addition, gaps in the unit are filled with an adhesive 5 of black silicon resin, or the like, to be firmly bonded to each other into an integrated unit.
Each rod lens 2 has a distribution of refractive index that even viewed in the cross-section, it decrease parabolically from maximum value in the center axis towards the pheriphery. Even if the ends (planes for incident and emerging light) are parallel, they have the same image-forming effect as the usual spherical lens.
The process of manufacturing the lens array unit 1 involves first a number of suitable prolonged glass rod made by the ion-exchange technique, and arranged in such way as closed to each other. Prolonged holding plates 3 are mounted on the upper and lower sides of the arrangement so as to hold it between them and spacers 4 were bilaterally mounted to assemble a long parent 10 of lens array unit as shown in FIG. 2A.
Subsequently, the resultant parent 10 of lens array unit is cut into a number of rod lens units 1 of a predetermined length with a diamond wheel cutter 6.
A plurality of lens array units 1 are piled vertically with respect to the lens rods, as shown in FIG. 2B, and held with a holder 7 and the lens ends are ground roughly with diamond pellets in a both-face lapping apparatus 8.
After finish grinding with lapping abrasive, polishing is made with polishing agent of free abrasive such as zirconia to impart the mirror surface smoothness.
The lens array units manufactured as described above have short focus and besides permit wide image as compared with the spherical lens system in the prior art. They therefore are used in optical systems for photocopying machine, photoprinter, etc. much contributing to compacting them.
As such an example, an LED photoprinter is schematically diagrammed in FIG. 3, in which reference characters designate as follows: 11 a photosensitive drum, 12 a charger, 13 an exposed head, 14 a developing device, 15 a transferring device, 16 a fixing unit, 17 a paper cassette and 18 a paper stacker.
The exposure head 13 incorporates a high density LED array and a lens array for condensing light on the peripheral surface of the drum 11. The LED emits light in accordance with electric signals sent in sequence to the exposure head 13 to form the electrostatic latent image on the drum 11, onto which toner is deposited, and then transferred to a paper.
Electrophotographic recording system including the exemplified LED photoprinter are provided with a charger for use in charging the photosensitive drum.
In the prior art optical device mentioned above however there are differences in workability among the rod lenses 2 of glass and the structural components 3, 4 and 5 surrounding them, which is made of different resins, this resulting in more rapid polishing of resins than glass by the effect of free abrasive. Conjointly with the influence of the elasticity of the polishing cloth, therefore, the end of the rod lens unit 1 can be polished with the result of irregularities between the end of the lens 2 projected and the area around it depressed as shown in FIG. 4, and this affects with deviation in image distance and occurrence of astigmatism.
There are additional encountered difficulties, for example, of requiring many steps of cutting, rough grinding, lapping, and finish polishing, and high cost of abrasive and others.
In addition, the high voltage charger of the LED photoprinter shown in FIG. 3 is responsible, by the electrical discharge from it, for production of much ozone (O.sub.3) from oxygen in the air. The ozone can react with water content in the air to produce nitric acid, which corrodes the surface of glass lens of the lens array, and in turn a reaction product with alkali metal contained in the lens glass separates, leading to making the lens surface cloudy and in turn reducing the optical performance.
A countermeasure against this is known which is to bake the lens array itself at about 200.degree. C. to previously separate alkali on the lens surface. This treatment alone however can effect not enough to prevent the deterioration.